Imaging device for use as radiation detector

ABSTRACT

An imaging device has a control block for offsetting signal levels and controlling the amplification factor for the signal levels within one frame. The imaging device can image a subject having low-illuminance and high-illuminance areas without reduction in the sensitivity, and allows a wide dynamic range and high sensitivity to be achieved real-time. The imaging device thus can image subjects of different illuminance levels ranging from a low-illuminance subject to a high-illuminance subject, and can differentiate small illuminance distributions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an imaging device, and moreparticularly to an infrared imaging device for use in temperaturemeasurements, etc.

2. Description of the Related Art

Infrared sensors convert an infrared radiation from the surface of asubject into an electric signal and displays the image of the subjectbased on the electric signal for measuring a temperature distribution,etc. of the surface of the subject. The infrared sensors are required tohave excellent temperature resolution so as to be able to distinguishsmall temperature differences and also to have a wide dynamic range soas to cover a wide range of temperatures that can be measured.

FIG. 1 of the accompanying drawings shows a conventional image sensorwhich includes at least bolometers 1 thermally isolated from a substrate(not shown) and serving as respective pixels, a vertical shift register9, a horizontal shift register 11, and integrating circuits 14, 15 forconverting a change in resistance into a change in voltage. Wheninfrared energy is radiated from a subject to be imaged, the radiatedinfrared energy is converged onto the imaging device by an opticalsystem, causing changes in the temperature of the bolometers 1, whichresult in changes in the resistance of the bolometers 1. The pixels aresuccessively selected by the vertical shift register 9 and thehorizontal shift register 11, and the changes in the resistance of thebolometers 1 are successively converted into electric signals by theintegrating circuits 14, 15. For further details, reference should bemade to Japanese laid-open patent publication 105794/1996.

Japanese laid-open patent publication 1989-102330 discloses anotherconventional imaging device which comprises an infrared sensor, asubtractor, a central temperature setting circuit, a gain selector, anda peak hold circuit. A temperature distribution of a subject to beimaged is converted into an electric signal by the infrared sensor.Signals from the pixels are successively read as with ordinarytelevision signals. The peak hold circuit holds maximum and minimumlevels of the pixel signals that have been read, and the gain of thegain selector is controlled to bring the signal levels into a desireddynamic range. The user of the imaging device manually sets the centraltemperature setting circuit to a desired central temperature.

A thermal infrared imaging device revealed in Japanese laid-open patentpublication 1996-46870 has an automatic level controller and anautomatic gain controller. The automatic level controller controls anoffset level to be added to a video signal such that the signal levelsof half of total pixels will be in a lower half of the dynamic range andthe signal levels of the remaining half of total pixels will be in anupper half of the dynamic range. The automatic gain controller controlsa gain such that the signal levels of the pixels will be in 10% to 90%of the dynamic range.

Japanese laid-open patent publication 137062/1995 shows an imagingdevice in which the difference between bright and dark levels ofcaptured images is increased by an automatic gain control (AGC) process.

An infrared imaging device disclosed in Japanese laid-open patentpublication 107074/1990 has a gain correcting circuit for correctingsensitivity variations of pixels of the infrared imaging device tosuppress fixed pattern noise (fixed DC level variations between thepixels) which is generated when the temperature of a subject beingimaged is greatly changed.

The conventional imaging devices referred to above, however, suffer thefollowing shortcomings:

(1) In the image sensor disclosed in Japanese laid-open patentpublication 105794/1996, any change in the temperature of the bolometersdue to incident infrared radiation is very small, and the image sensoritself has a very large dynamic range. For example, when a subjecthaving a temperature difference of 1° C. is imaged, the temperature ofthe bolometers changes only by 2×10⁻³ ° C. Even when a subject having atemperature of several hundred degrees centigrade is imaged, any changein the temperature of the bolometers does not exceed 1° C. Therefore,the dynamic range of the thermal infrared imaging device is determinedby the dynamic range of an amplifier, etc. which amplifies a videosignal from the image sensor.

For imaging a subject having a low-illuminance area and ahigh-illuminance area, since the amplification factor of the amplifieris lowered so that signal levels from the low-illuminance area and thehigh-illuminance area will enter the dynamic range, the temperatureresolution is lowered, resulting in a failure to observe a smallilluminance distribution on the surfaces of the subjects.

(2) The imaging devices disclosed in Japanese laid-open patentpublications 102330/1980, 46870/1996, and 137062/1995 have the sameproblems as described above because they have AGC circuits or similarcircuits and the amplification factor of the amplifier and the offsetlevel for the video signal are controlled by the AGC circuits or similarcircuits such that the signal levels of an overall subject to be imagedwill enter the dynamic range.

The infrared imaging device disclosed in Japanese laid-open patentpublication 107074/1990 fails to solve the above problems because itcorrects the gain variations of individual pixels for producing uniformimages.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an imagingdevice capable of imaging subjects ranging from a low-illuminancesubject to a high-illuminance subject and of distinguishing slightilluminance distributions.

In the arrangement of the present invention, since signal levels andamplification factors for the signal levels are changed in one framedepending on the intensities of light incident upon the detecting means,a subject having low-illuminance and high-illuminance areas can beimaged under signal processing conditions optimum for thelow-illuminance and high-illuminance areas.

A frequency map of the intensities of light incident upon the detectingmeans is generated, and signal levels in the frequency map are convertedinto parts having frequencies greater than a given level, thus providingdata required to determine offset levels and amplification factors.

When offset levels and amplification factors are changed, those imageareas where they are changed may be displayed in colors or in differentdisplay windows or monitor display units for thereby allowing the userto distinguish the displayed areas.

The above and other subjects, features, and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings which illustrate examples of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram, partly in block form, of a conventionalinfrared imaging device;

FIG. 2 is a block diagram of an imaging device according to the presentinvention;

FIG. 3 is a block diagram of a control block for use in the imagingdevice shown in FIG. 2;

FIG. 4 is a block diagram of another control block for use in theimaging device shown in FIG. 2;

FIG. 5a is a diagram showing intensities of light from a subject,illustrative of the manner in which a control circuit shown in FIG. 3operates;

FIG. 5b is a diagram showing a frequency map of the intensities of lightshown in FIG. 5a;

FIG. 5c is a diagram showing signals produced when the frequency mapshown in FIG. 5b is integrated with respect to signal levels;

FIG. 5d is a diagram showing signal parts converted from the signallevels shown in FIG. 5c using a threshold value;

FIG. 5e is a diagram showing groups into which the signal parts shown inFIG. 5d are classified;

FIG. 5f is a diagram showing minimum and maximum values of signal levelranges in the groups shown in FIG. 5e;

FIG. 6 is a block diagram illustrative of a specific mode of operationof a display circuit of the imaging device shown in FIG. 2;

FIG. 7 is a block diagram illustrative of another specific mode ofoperation of the display circuit of the imaging device shown in FIG. 2;

FIG. 8 is a block diagram illustrative of still another specific mode ofoperation of the display circuit of the imaging device shown in FIG. 2;

FIG. 9 is a block diagram illustrative of yet still another specificmode of operation of the display circuit of the imaging device shown inFIG. 2; and

FIG. 10 is a detailed circuit diagram, partly in block form, of asubtractor, a variable-gain amplifier, and related circuits of theimaging device shown in FIG. 2.

FIG. 11 is a block diagram of an imaging device according to the presentinvention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 2, an imaging device according to the present inventioncomprises a sensor 101 as detecting means for detecting infrared energyradiated from a subject, converting the detected infrared energy into anelectric signal, and outputting the electric signal, an amplifier 102 asfirst amplifying means for amplifying the electric signal from thesensor 101 and outputting the amplified electric signal, a control block106 for determining low-illuminance and high-illuminance areas of thesubject based on a relatively less amplified electric signal of theelectric signal outputted from the amplifier 102, generating an offsetlevel, a designated amplification factor, and a display switching signalwithin one frame, a subtractor 103 for subtracting the offset leveloutputted from the control block 106 from the electric signal outputtedfrom the amplifier 102, a variable-gain amplifier 104 for amplifying asignal outputted from the subtractor 103 based on the designatedamplification factor outputted from the control block 106 and outputtingthe amplified signal, and a display circuit 105 as display means forselectively displaying the low-illuminance and high-illuminance areas ofthe subject and an intermediate-illuminance area, if any, of the subjectbased on the signal outputted from the variable-gain amplifier 104according to the display switching signal outputted from the controlblock 106. The subtractor 103 may be replaced with an adder if the signof the offset level is changed.

Operation of the imaging device will be described below.

When infrared energy is radiated from the subject, the radiated infraredenergy is detected by the sensor 101, which converts the detectedinfrared energy into an electric signal and outputs the electric signal.

The amplifier 102 amplifies the electric signal outputted from thesensor 101 with a predetermined amplification factor, and outputs theamplified electric signal to the subtractor 103 and the control block106.

The control block 106 determines low-illuminance and high-illuminanceareas of the subject based on a relatively less amplified electricsignal of the electric signal outputted from the amplifier 102,generates within one frame an offset level as a quantity to besubtracted by the subtractor 103, a designated amplification factor asan amplification factor for use in the variable-gain amplifier 104, anda display switching signal for selecting images to be displayed in thedisplay circuit 105, and outputs the offset level, the designatedamplification factor, and the display switching signal in a real-timemanner.

The subtractor 103 subtracts the offset level outputted from the controlblock 106 from the electric signal outputted from the amplifier 102.

The variable-gain amplifier 104 amplifies a signal outputted from thesubtractor 103 based on the designated amplification factor outputtedfrom the control block 106 and outputs the amplified signal.

The display circuit 105 selectively displays the low-illuminance andhigh-illuminance areas of the subject and an intermediate-illuminancearea, if any, of the subject based on the signal outputted from thevariable-gain amplifier 104 according to the display switching signaloutputted from the control block 106.

As shown in FIG. 3, one arrangement of the control block 106 comprisesan A/D converter 201 for converting the electric signal outputted fromthe sensor 101 into a digital signal, a memory 202 for storing data tobe used in the generation of a frequency map and other processing, and acontrol circuit 203 for generating a frequency map of intensities oflight (magnitudes of video signals) incident upon the sensor 101,calculating an offset level and an amplification factor optimum for thelow-illuminance and high-illuminance areas depending on thelow-illuminance and high-illuminance areas, and outputting the offsetlevel and the amplification factor that are calculated. Calculations inthe control circuit 203 are carried out on the data of a certain frame,and are reflected in the data of a next frame.

As shown in FIG. 4, another arrangement of the control block comprises acomparator 301 for determining low-illuminance and high-illuminanceareas, and a control circuit 302 for generating an offset level and anamplification factor based on the result from the comparator 301 andoutputting the offset level and the amplification factor that aregenerated. The control block shown in FIG. 4 allows the decision about acertain pixel to be instantly reflected in the control of that pixel.While the control block shown in FIG. 4 is simple in circuitarrangement, it is unable to perform complex control processes forautomatically changing offset levels and amplification factors dependingon the subject.

Operation of the control circuit 203 shown in FIG. 3 will be describedbelow.

It is assumed that there is a subject having both low-illuminance andhigh-illuminance areas mixed together as shown in FIG. 5a.

If the subject shown in FIG. 5a is imaged by the conventional imagingdevices, then since the signal level of the high-illuminance area wouldtend to fall out of the dynamic range, the aperture of the opticalsystem is adjusted or the AGC circuit is operated to bring the signallevel of the high-illuminance area into the dynamic range, but suchprocesses result in an overall reduction in the sensitivity and thetemperature resolution. The dynamic range referred to above mayrepresent the dynamic range of the sensor, the dynamic range of theamplifier, the dynamic range of the A/D converter, or the dynamic rangeof a display monitor unit. With the thermal infrared sensor, however,the dynamic range of the sensor is not limitative as it is very large asdescribed above.

An infrared imaging device, for example, usually has a temperatureresolution of about 0.1° C. at minimum, and a dynamic range of at most100° C. Therefore, when a hot object having a temperature of severalhundred degrees centigrade is to be imaged as a subject, the overallsensitivity, i.e., the temperature resolution of the infrared imagingdevice has to be lowered.

To this end, as shown in FIG. 5b, pixel signals from the imaged subjectare read, and a frequency map is generated which has a horizontal axisrepresenting intensities of incident light (magnitudes of video signals)and a vertical axis representing frequencies with respect to the lightintensities. The frequency map shown in FIG. 5b contains level peakscorresponding to the low-illuminance area and level peaks correspondingto the high-illuminance area.

A process of generating the frequency map will be described below withreference to FIGS. 1 through 5a-5f.

When the amplified electric signal from the amplifier 101 is supplied tothe control block 106, the electric signal levels of respective pixels(intensities of the incident light) are successively converted intocorresponding digital values by the A/D converter 201. The digitalvalues have signal levels which may range from 0 to 255, for example.

In the control circuit 203, the digital values from the A/D converter201 are stored as data at addresses in the memory 202, and the data atthe addresses are incremented. When a signal level of 0 is supplied fromthe A/D converter 201 to the memory 202, "1" is added to the data at anaddress "0" in the memory 202, and the resultant data is returned to theaddress "0". Successive signal levels supplied from the A/D converter201 are similarly stored at the respective addresses in the memory 202.The data stored in the memory 202 are cleared in every frame. In thismanner, a frequency map of the signal levels ranging from 0 to 255 isgenerated. While the digital values are illustrated as having signallevels ranging from 0 to 255, the digital values may be represented bymore bits. The horizontal axis of the frequency map may not necessarilyrepresent as many bits as the bits of the signal levels of the digitalvalues, but may represent high-order bits of the signal levels of thedigital values, thereby reducing the number of bits on the horizontalaxis.

In the frequency map shown in FIG. 5b, the signal levels (incident lightintensities) suffer small variations or fluctuations due to noise or thelike. If such small variations or fluctuations were left unremoved, thenthey would possibly impair subsequent signal processing. Therefore, itis preferable to integrate the frequency map with respect to the signallevels (incident light intensities) as shown in FIG. 5c.

While various techniques are available for numerical integration, onerelatively simple integrating process will be described below.

A frequency map prior to being integrated and a frequency map afterbeing integrated are stored in separate memory areas, and the frequencyof a certain signal level prior to being integrated and the frequenciesof signal levels immediately before and after the certain signal levelare averaged, after which the averaged value is stored in the frequencymap after being integrated.

The above process is carried out on all signal levels, therebycompleting the integration of the frequency map in its entirety.

Rather than using a certain signal level and signal levels immediatelybefore and after the certain signal level, such a certain signal leveland a plurality of signal levels in the vicinity of the certain signallevel may be averaged. The signal levels in the vicinity of the certainsignal level may be weighted by a coefficient depending on the distancefrom the certain signal level.

Then, as shown in FIG. 5d, in order to avoid the influence of noise orthe like, a certain threshold value is established in the frequency map,and the signal levels beyond the threshold value are converted intoparts. For example, such parts are marked with A, B, C, D, E, and thesignal level ranges of the parts A, B, C, D, E are stored in a memoryarea.

When the signal levels beyond the threshold value are converted intoparts, since the same subject may be divided into a plurality of parts,as shown in FIG. 5d, the parts are preferably classified into groups asshown in FIG. 5e. In the illustrated example, the parts A, B, C areclassified into a low-illuminance group, and the parts D, E areclassified into a high-illuminance group.

The signal level ranges of the parts A, B, C, D, E have already beendetermined in the step shown in FIG. 5d. Based on the determined signallevel ranges, those parts which are spaced by a distance smaller than acertain distance level are classified into the same group. The signallevel ranges of the groups are stored in a memory area.

In FIG. 5e, the parts are classified into the two groups, i.e., thelow-illuminance group and the high-illuminance group. However, somesubjects may generate more groups of parts converted from signal levels.Though it is possible to give optimum offset levels and amplificationfactors to all such groups, it will require complex display means todisplay all the groups.

There are available several methods of displaying, not all of aplurality of groups, but certain ones of the groups.

According to one method, for example, the total frequency of signallevels in each of the groups is calculated, and several groups with highfrequencies are displayed. According to another process, a certainsignal level (incident light intensity) or certain signal levels(incident light intensities) are established, and a group or groupsclosest to the certain signal level or levels are displayed. Anothermethod determines a certain location at the center of the displayscreen, for example, and displays groups that contain the location.

An infrared imaging device, for example, is required to display at alltimes regions near normal temperature for positioning purpose.Therefore, the infrared imaging device employs a process of necessarilyselecting groups near normal temperature and selecting one other groupfrom those groups which have high total frequencies, which have acertain temperature, or which belong to a certain location.

Subsequently, an offset level and an amplification factor which areoptimum for each of the selected groups are determined.

As shown in FIG. 5f, an offset level for a certain group is determinedas (max-min)/2 where max represents the maximum value of signal levelranges of the group and min the minimum value of signal level ranges ofthe group.

The offset level is supplied to the subtractor 103, which subtracts theoffset level from a video signal outputted from the amplifier 102.

Since the offset level is changed real-time in one frame, signal levelsfrom a high-illuminance area which would otherwise be excluded from thedynamic range will fall in the dynamic range without lowering thesensitivity.

Various methods are available for establishing a reference level forvideo signals.

Usually, the sensor 101 often incorporates an optically black pixel OBwhich is insensitive to light, and the level of a signal generated bythe optically black pixel OB is used as a reference level.

In an infrared imaging device, the pixel signals vary about the signallevel of the optically black pixel OB. In a visible imaging device,since the optically black pixel OB is the darkest pixel, the pixelsignals do not vary about the signal level of the optically black pixelOB, but vary about a level higher than the signal level of the opticallyblack pixel OB. The signal level of the optically black pixel OB may beestablished as a reference level.

Rather than the signal level of the optically black pixel OB, theaverage of the signal levels from all the pixels may be established as areference level.

At any rate, a reference level may be established as an arbitrary level.While the value of the offset level (max-min)/2 may be variabledepending on the reference level, this is caused by the reference level,and the offset level (max-min)/2 still remains to be subtracted from thevideo signal outputted from the amplifier 102.

The amplification factor optimum for each of the groups is establishedgenerally in proportion to 1/(max-min), and outputted to thevariable-gain amplifier 104.

The amplification factor may be very large depending on the subject.When the video signal is amplified beyond a certain level, the neteffect is that only noise contained in the video signal becomesconspicuous. For this reason, a maximum amplification factor may beestablished, and any amplification factors beyond a certain value may beset to the maximum amplification factor.

As described above, the amplification factor is changed real-time in oneframe for thereby establishing amplification factors optimum for groupsor objects having arbitrary levels of brightness.

While the process of determining offset levels and amplification factorsoptimum for groups has been described above, parts may be used insteadof groups if the parts are not classified into groups.

In certain applications, amplification factors may be establishedmanually and offset levels may be established automatically, orconversely offset levels may be established manually and amplificationfactors may be established automatically, or further alternatively, bothoffset levels and amplification factors may be established manually.These processes of establishing offset levels and amplification factorsare based on the principles of the present invention insofar as theoffset levels and the amplification factors are changed in one frame.

According to the above process of generating a frequency map, thefrequency map is generated using the data of all pixels in a certainframe. However, it is not necessary to generate a frequency map in everyframe. Though calculations may be made in every frame, since abruptchanges in offset levels and amplification factors tend to impair thequality of displayed images, the offset levels and the amplificationfactors should preferably be changed with a long time constant. Forexample, offset levels and amplification factors calculated in everyframe may be integrated over a plurality of frames.

As described above, offset levels and amplification factors are changeddepending on low-illuminance and high-illuminance areas. Consequently,if the video signal outputted from the variable-gain amplifier 104 isdirectly displayed, incident lights having different intensities will bedisplayed as signals of the same level.

To avoid the above drawback, the process of displaying images is changedat the same time that offset levels and amplification factors arechanged.

The control block 106 changes offset levels and amplification factors atthe time when the signal is outputted from the low-illuminance area, andindicates to the display circuit 105 that the signal is being outputtedfrom the low-illuminance area. The display circuit 105 sets a displaymode to a black-and-white gray scale display mode, for example (see FIG.6). Similarly, the control block 106 changes offset levels andamplification factors at the time when the signal is outputted from thehigh-illuminance area, and indicates to the display circuit 105 that thesignal is being outputted from the high-illuminance area. The displaycircuit 105 sets a display mode to a color code display mode, forexample. In the color code display mode, as shown in FIG. 6, blue (B),green (G), and red (R) levels are appropriately changed depending on thesignal level. This allows the observer to distinguish mixed videosignals displayed on one display screen.

As shown in FIG. 7, the display circuit may display a group in ablack-and-white gray scale display mode, another group in a bluegradation display mode with different densities of blue, still anothergroup in a green gradation display mode, and still yet another group ina red gradation display mode. This also allows the observer todistinguish mixed video signals displayed on one display screen. Thenumber of groups that can be displayed may be increased by usinggradation display modes of other additional colors.

In FIG. 8, the display circuit has a display data memory 702 for storingdata to be displayed in display windows on the display screen, and awrite control circuit 701 for storing data into the display data memory702.

The display circuit shown in FIG. 8 displays video signals of differentgroups in the respective display windows on the display screen.Therefore, the displayed video signals of different groups will notpossibly be recognized as having the same signal level.

In FIG. 9, there are a plurality of display monitor units and thedisplay circuit has a plurality of video output circuits (NTSC) fordisplaying images of different groups on the display monitor units,respectively. Areas other than the groups are fixed to a certain signallevel.

FIG. 10 shows in detail the subtractor 103, the variable-gain amplifier104, and related circuits of the imaging device shown in FIG. 2. In FIG.10, the signal level of the optically black pixel OB is clamped to alevel Vref by an OB clamp circuit 901 for thereby setting the referencelevel for video signals to the level Vrel. An operational amplifier 902as an adder operates as a subtractor when an inverted offset signal isoutputted from a control block 904. An operational amplifier 903 servesas a variable-gain amplifier whose amplification factor is controlled byselecting a resistance of a feedback resistance block 905 based on asignal outputted from the control block 904.

With the present invention, the imaging device has a control block foroffsetting a signal level in one frame and controlling the amplificationfactor of the signal level based on the intensity of incident light forproducing a video signal. Therefore, a subject containinglow-illuminance and high-illuminance areas can be imaged without areduction in the sensitivity, and hence a wide dynamic range and a highsensitivity level can be achieved real-time.

If the principles of the present invention are applied to a thermalinfrared imaging device, then a very wide dynamic range of the sensor ofthe thermal infrared imaging device can effectively be utilized. When afurnace which requires delicate temperature control is to be observedfor temperatures, it has heretofore been impossible to measure thetemperatures of the furnace with high resolution because signals basedon infrared energy radiated from the furnace need to be brought into thedynamic range by lowering the sensitivity. According to the presentinvention, however, the imaging device can measure the temperatures ofthe furnace with high resolution, and can observe a plurality oftemperature areas simultaneously. For example, the imaging device allowsa normal-temperature region and the furnace to be observed at the sametime for thereby permitting the user to observe the furnace temperatureswhile observing surrounding temperature conditions.

As shown in FIG. 11, one arrangement of the control block 106 comprisessection of offsetting 11 for offsetting the electric signal outputtedfrom the amplifier 102 by predetermined quantity, section of controlling12 for controlling the variable amplification factor of thevariable-again amplifier 104 within one frame of the electric signal,section of generating 13 for generating a frequency map of intensitiesof light incident upon the sensor 101, section of integrating 14 forintegrating frequency map with respect to intensities of light incidentupon the sensor 101, section of converting 15 for converting frequencymap into parts having frequencies greater than a predetermined level andintensities adjacent to each other, section of classifying 16 forclassifying the parts into groups according to intensities of lightincident upon the sensor 101.

While preferred embodiments of the present invention have been describedusing specific terms, such description is for illustrative purposesonly, and it is to be understood that changes and variations may be madewithout departing from the spirit or scope of the following claims.

What is claimed is:
 1. An imaging device comprising:detecting means fordetecting a radiation radiated from a subject, converting the detectedradiation into an electric signal, and outputting the electric signal;first amplifying means for amplifying the electric signal from saiddetecting means with a predetermined amplification factor and outputtingthe amplified electric signal; second amplifying means for amplifyingthe electric signal from said first amplifying means with a variableamplification factor and outputting the amplified electric signal;display means for displaying an image of said subject based on theelectric signal outputted from said second amplifying means; and controlmeans for controlling the variable amplification factor of said secondamplifying means based on the electric signal outputted from said firstamplifying means.
 2. An imaging device according to claim 1, whereinsaid control means comprises means for controlling the variableamplification factor of said second amplifying means within one frame ofthe electric signal.
 3. An imaging device comprising:detecting means fordetecting a radiation radiated from a subject, converting the detectedradiation into an electric signal, and outputting the electric signal;first amplifying means for amplifying the electric signal from saiddetecting means with a predetermined amplification factor and outputtingthe amplified electric signal; second amplifying means for amplifyingthe electric signal from said first amplifying means with a variableamplification factor and outputting the amplified electric signal;display means for displaying an image of said subject based on theelectric signal outputted from said second amplifying means; and controlmeans for offsetting the electric signal outputted from said firstamplifying means by a predetermined quantity and controlling thevariable amplification factor of said second amplifying means based onthe electric signal outputted from said first amplifying means.
 4. Animaging device according to claim 3, wherein said control meanscomprises means for offsetting the electric signal outputted from saidfirst amplifying means by said predetermined quantity and controllingthe variable amplification factor of said second amplifying means withinone frame of the electric signal.
 5. An imaging device according toclaim 3, wherein said control means comprises means for generating afrequency map of intensities of light incident upon said detectingmeans.
 6. An imaging device comprising:detecting means for detecting aradiation radiated from a subject, converting the detected radiationinto an electric signal, and outputting the electric signal; firstamplifying means for amplifying the electric signal from said detectingmeans with a predetermined amplification factor and outputting theamplified electric signal; second amplifying means for amplifying theelectric signal from said first amplifying means with a variableamplification factor and outputting the amplified electric signal;display means for displaying an image of said subject based on theelectric signal outputted from said second amplifying means; and controlmeans for offsetting the electric signal outputted from said firstamplifying means by a predetermined quantity and controlling thevariable amplification factor of said second amplifying means based onthe electric signal outputted from said first amplifying means; saidcontrol means comprising:means for offsetting the electric signaloutputted from said first amplifying means by said predeterminedquantity and controlling the variable amplification factor of saidsecond amplifying means within one frame of the electric signal; andmeans for generating a frequency map of intensities of light incidentupon said detecting means.
 7. An imaging device according to claim 6,wherein said control means comprises means for integrating saidfrequency map with respect to intensities of light incident upon saiddetecting means.
 8. An imaging device comprising:detecting means fordetecting a radiation radiated from a subject, converting the detectedradiation into an electric signal, and outputting the electric signal;first amplifying means for amplifying the electric signal from saiddetecting means with a predetermined amplification factor and outputtingthe amplified electric signal; second amplifying means for amplifyingthe electric signal from said first amplifying means with a variableamplification factor and outputting the amplified electric signal;display means for displaying an image of said subject based on theelectric signal outputted from said second amplifying means; and controlmeans for offsetting the electric signal outputted from said firstamplifying means by a predetermined quantity and controlling thevariable amplification factor of said second amplifying means based onthe electric signal outputted from said first amplifying means; saidcontrol means comprising:means for generating a frequency map ofintensities of light incident upon said detecting means; and means forintegrating said frequency map with respect to intensities of lightincident upon said detecting means.
 9. An imaging devicecomprising:detecting means for detecting a radiation radiated from asubject, converting the detected radiation into an electric signal, andoutputting the electric signal; first amplifying means for amplifyingthe electric signal from said detecting means with a predeterminedamplification factor and outputting the amplified electric signal;second amplifying means for amplifying the electric signal from saidfirst amplifying means with a variable amplification factor andoutputting the amplified electric signal; display means for displayingan image of said subject based on the electric signal outputted fromsaid second amplifying means; and control means for offsetting theelectric signal outputted from said first amplifying means by apredetermined quantity and controlling the variable amplification factorof said second amplifying means based on the electric signal outputtedfrom said first amplifying means; said control means comprising:meansfor generating a frequency map of intensities of light incident uponsaid detecting means; and means for converting said frequency map intoparts having frequencies greater than a predetermined level andintensities adjacent to each other.
 10. An imaging device according toclaim 9, wherein said control means comprises means for offsetting theelectric signal outputted from said first amplifying means by(max-min)/2 where max represents a maximum intensity of incident lightin the parts and min represents a minimum intensity of incident light inthe parts.
 11. An imaging device according to claim 9, wherein saidcontrol means comprises means for controlling the variable amplificationfactor of said second amplifying means in proportion to 1/(max-min)where max represents a maximum intensity of incident light in the partsand min represents a minimum intensity of incident light in the parts.12. An imaging device according to claim 9, wherein said display meanscomprises means for displaying pixels in a selected one of the parts ina monochromatic gray scale mode and displaying pixels in the other partsin a color display mode.
 13. An imaging device according to claim 9,wherein said display means has a plurality of windows and comprisesmeans for displaying pixels in the parts in said windows, respectively.14. An imaging device according to claim 9, wherein said display meanshas a plurality of display monitor units and comprises means fordisplaying pixels in the parts in said display monitor units,respectively.
 15. An imaging device comprising:detecting means fordetecting a radiation radiated from a subject, converting the detectedradiation into an electric signal, and outputting the electric signal;first amplifying means for amplifying the electric signal from saiddetecting means with a predetermined amplification factor and outputtingthe amplified electric signal; second amplifying means for amplifyingthe electric signal from said first amplifying means with a variableamplification factor and outputting the amplified electric signal;display means for displaying an image of said subject based on theelectric signal outputted from said second amplifying means; and controlmeans for offsetting the electric signal outputted from said firstamplifying means by a predetermined quantity and controlling thevariable amplification factor of said second amplifying means based onthe electric signal outputted from said first amplifying means; saidcontrol means comprising:means for offsetting the electric signaloutputted from said first amplifying means by said predeterminedquantity and controlling the variable amplification factor of saidsecond amplifying means within one frame of the electric signal; meansfor generating a frequency map of intensities of light incident uponsaid detecting means; and means for converting said frequency map intoparts having frequencies greater than a predetermined level andintensities adjacent to each other.
 16. An imaging device according toclaim 15, wherein said control means comprises means for offsetting theelectric signal outputted from said first amplifying means by(max-min)/2 where max represents a maximum intensity of incident lightin the parts and min represents a minimum intensity of incident light inthe parts.
 17. An imaging device according to claim 15, wherein saidcontrol means comprises means for controlling the variable amplificationfactor of said second amplifying means in proportion to 1/(max-min)where max represents a maximum intensity of incident light in the partsand min represents a minimum intensity of incident light in the parts.18. An imaging device according to claim 15, wherein said display meanscomprises means for displaying pixels in a selected one of the parts ina monochromatic gray scale mode and displaying pixels in the other partsin a color display mode.
 19. An imaging device according to claim 15,wherein said display means has a plurality of windows and comprisesmeans for displaying pixels in the parts in said windows, respectively.20. An imaging device according to claim 15, wherein said display meanshas a plurality of display monitor units and comprises means fordisplaying pixels in the parts in said display monitor units,respectively.
 21. An imaging device comprising:detecting means fordetecting a radiation radiated from a subject, converting the detectedradiation into an electric signal, and outputting the electric signal;first amplifying means for amplifying the electric signal from saiddetecting means with a predetermined amplification factor and outputtingthe amplified electric signal; second amplifying means for amplifyingthe electric signal from said first amplifying means with a variableamplification factor and outputting the amplified electric signal;display means for displaying an image of said subject based on theelectric signal outputted from said second amplifying means; and controlmeans for offsetting the electric signal outputted from said firstamplifying means by a predetermined quantity and controlling thevariable amplification factor of said second amplifying means based onthe electric signal outputted from said first amplifying means; saidcontrol means comprising:means for generating a frequency map ofintensities of light incident upon said detecting means; means forintegrating said frequency map with respect to intensities of lightincident upon said detecting means; and means for converting saidfrequency map into parts having frequencies greater than a predeterminedlevel and intensities adjacent to each other.
 22. An imaging deviceaccording to claim 21, wherein said control means comprises means foroffsetting the electric signal outputted from said first amplifyingmeans by (max-min)/2 where max represents a maximum intensity ofincident light in the parts and min represents a minimum intensity ofincident light in the parts.
 23. An imaging device according to claim21, wherein said control means comprises means for controlling thevariable amplification factor of said second amplifying means inproportion to 1/(max-min) where max represents a maximum intensity ofincident light in the parts and min represents a minimum intensity ofincident light in the parts.
 24. An imaging device according to claim21, wherein said display means comprises means for displaying pixels ina selected one of the parts in a monochromatic gray scale mode anddisplaying pixels in the other parts in a color display mode.
 25. Animaging device according to claim 21, wherein said display means has aplurality of windows and comprises means for displaying pixels in theparts in said windows, respectively.
 26. An imaging device according toclaim 21, wherein said display means has a plurality of display monitorunits and comprises means for displaying pixels in the parts in saiddisplay monitor units, respectively.
 27. An imaging devicecomprising:detecting means for detecting a radiation radiated from asubject, converting the detected radiation into an electric signal, andoutputting the electric signal; first amplifying means for amplifyingthe electric signal from said detecting means with a predeterminedamplification factor and outputting the amplified electric signal;second amplifying means for amplifying the electric signal from saidfirst amplifying means with a variable amplification factor andoutputting the amplified electric signal; display means for displayingan image of said subject based on the electric signal outputted fromsaid second amplifying means; and control means for offsetting theelectric signal outputted from said first amplifying means by apredetermined quantity and controlling the variable amplification factorof said second amplifying means based on the electric signal outputtedfrom said first amplifying means; said control means comprising:meansfor offsetting the electric signal outputted from said first amplifyingmeans by said predetermined quantity and controlling the variableamplification factor of said second amplifying means within one frame ofthe electric signal; means for generating a frequency map of intensitiesof light incident upon said detecting means; means for integrating saidfrequency map with respect to intensities of light incident upon saiddetecting means; and means for converting said frequency map into partshaving frequencies greater than a predetermined level and intensitiesadjacent to each other.
 28. An imaging device according to claim 27,wherein said control means comprises means for offsetting the electricsignal outputted from said first amplifying means by (max-min)/2 wheremax represents a maximum intensity of incident light in the parts andmin represents a minimum intensity of incident light in the parts. 29.An imaging device according to claim 27, wherein said control meanscomprises means for controlling the variable amplification factor ofsaid second amplifying means in proportion to 1/(max-min) where maxrepresents a maximum intensity of incident light in the parts and minrepresents a minimum intensity of incident light in the parts.
 30. Animaging device according to claim 27, wherein said display meanscomprises means for displaying pixels in a selected one of the parts ina monochromatic gray scale mode and displaying pixels in the other partsin a color display mode.
 31. An imaging device according to claim 27,wherein said display means has a plurality of windows and comprisesmeans for displaying pixels in the parts in said windows, respectively.32. An imaging device according to claim 27, wherein said display meanshas a plurality of display monitor units and comprises means fordisplaying pixels in the parts in said display monitor units,respectively.
 33. An imaging device comprising:detecting means fordetecting a radiation radiated from a subject, converting the detectedradiation into an electric signal, and outputting the electric signal;first amplifying means for amplifying the electric signal from saiddetecting means with a predetermined amplification factor and outputtingthe amplified electric signal; second amplifying means for amplifyingthe electric signal from said first amplifying means with a variableamplification factor and outputting the amplified electric signal;display means for displaying an image of said subject based on theelectric signal outputted from said second amplifying means; and controlmeans for offsetting the electric signal outputted from said firstamplifying means by a predetermined quantity and controlling thevariable amplification factor of said second amplifying means based onthe electric signal outputted from said first amplifying means; saidcontrol means comprising:means for generating a frequency map ofintensities of light incident upon said detecting means; means forconverting said frequency map into parts having frequencies greater thana predetermined level and intensities adjacent to each other; and meansfor classifying the parts into groups according to intensities of lightincident upon said detecting means.
 34. An imaging device according toclaim 33, wherein said control means comprises means for offsetting theelectric signal outputted from said first amplifying means by(max-min)/2 where max represents a maximum intensity of incident lightin the parts or the groups and min represents a minimum intensity ofincident light in the parts or the groups.
 35. An imaging deviceaccording to claim 33, wherein said control means comprises means forcontrolling the variable amplification factor of said second amplifyingmeans in proportion to 1/(max-min) where max represents a maximumintensity of incident light in the parts or the groups and minrepresents a minimum intensity of incident light in the parts or thegroups.
 36. An imaging device according to claim 33, wherein saiddisplay means comprises means for displaying pixels in a selected one ofthe parts or the groups in a monochromatic gray scale mode anddisplaying pixels in the other parts or the other groups in a colordisplay mode.
 37. An imaging device according to claim 33, wherein saiddisplay means has a plurality of windows and comprises means fordisplaying pixels in the parts or the groups in said windows,respectively.
 38. An imaging device according to claim 33, wherein saiddisplay means has a plurality of display monitor units and comprisesmeans for displaying pixels in the parts or the groups in said displaymonitor units, respectively.
 39. An imaging device comprising:detectingmeans for detecting a radiation radiated from a subject, converting thedetected radiation into an electric signal, and outputting the electricsignal; first amplifying means for amplifying the electric signal fromsaid detecting means with a predetermined amplification factor andoutputting the amplified electric signal; second amplifying means foramplifying the electric signal from said first amplifying means with avariable amplification factor and outputting the amplified electricsignal; display means for displaying an image of said subject based onthe electric signal outputted from said second amplifying means; andcontrol means for offsetting the electric signal outputted from saidfirst amplifying means by a predetermined quantity and controlling thevariable amplification factor of said second amplifying means based onthe electric signal outputted from said first amplifying means; saidcontrol means comprising:means for offsetting the electric signaloutputted from said first amplifying means by said predeterminedquantity and controlling the variable amplification factor of saidsecond amplifying means within one frame of the electric signal; meansfor generating a frequency map of intensities of light incident uponsaid detecting means; means for converting said frequency map into partshaving frequencies greater than a predetermined level and intensitiesadjacent to each other; and means for classifying the parts into groupsaccording to intensities of light incident upon said detecting means.40. An imaging device according to claim 39, wherein said control meanscomprises means for offsetting the electric signal outputted from saidfirst amplifying means by (max-min)/2 where max represents a maximumintensity of incident light in the parts or the groups and minrepresents a minimum intensity of incident light in the parts or thegroups.
 41. An imaging device according to claim 39, wherein saidcontrol means comprises means for controlling the variable amplificationfactor of said second amplifying means in proportion to 1/(max-min)where max represents a maximum intensity of incident light in the partsor the groups and min represents a minimum intensity of incident lightin the parts or the groups.
 42. An imaging device according to claim 39,wherein said display means comprises means for displaying pixels in aselected one of the parts or the groups in a monochromatic gray scalemode and displaying pixels in the other parts or the other groups in acolor display mode.
 43. An imaging device according to claim 39, whereinsaid display means has a plurality of windows and comprises means fordisplaying pixels in the parts or the groups in said windows,respectively.
 44. An imaging device according to claim 39, wherein saiddisplay means has a plurality of display monitor units and comprisesmeans for displaying pixels in the parts or the groups in said displaymonitor units, respectively.
 45. An imaging device comprising:detectingmeans for detecting a radiation radiated from a subject, converting thedetected radiation into an electric signal, and outputting the electricsignal; first amplifying means for amplifying the electric signal fromsaid detecting means with a predetermined amplification factor andoutputting the amplified electric signal; second amplifying means foramplifying the electric signal from said first amplifying means with avariable amplification factor and outputting the amplified electricsignal; display means for displaying an image of said subject based onthe electric signal outputted from said second amplifying means; andcontrol means for offsetting the electric signal outputted from saidfirst amplifying means by a predetermined quantity and controlling thevariable amplification factor of said second amplifying means based onthe electric signal outputted from said first amplifying means; saidcontrol means comprising:means for generating a frequency map ofintensities of light incident upon said detecting means; means forintegrating said frequency map with respect to intensities of lightincident upon said detecting means; means for converting said frequencymap into parts having frequencies greater than a predetermined level andintensities adjacent to each other; and means for classifying the partsinto groups according to intensities of light incident upon saiddetecting means.
 46. An imaging device according to claim 45, whereinsaid control means comprises means for offsetting the electric signaloutputted from said first amplifying means by (max-min)/2 where maxrepresents a maximum intensity of incident light in the parts or thegroups and min represents a minimum intensity of incident light in theparts or the groups.
 47. An imaging device according to claim 45,wherein said control means comprises means for controlling the variableamplification factor of said second amplifying means in proportion to1/(max-min) where max represents a maximum intensity of incident lightin the parts or the groups and min represents a minimum intensity ofincident light in the parts or the groups.
 48. An imaging deviceaccording to claim 45, wherein said display means comprises means fordisplaying pixels in a selected one of the parts or the groups in amonochromatic gray scale mode and displaying pixels in the other partsor the other groups in a color display mode.
 49. An imaging deviceaccording to claim 45, wherein said display means has a plurality ofwindows and comprises means for displaying pixels in the parts or thegroups in said windows, respectively.
 50. An imaging device according toclaim 45, wherein said display means has a plurality of display monitorunits and comprises means for displaying pixels in the parts or thegroups in said display monitor units, respectively.
 51. An imagingdevice comprising:detecting means for detecting a radiation radiatedfrom a subject, converting the detected radiation into an electricsignal, and outputting the electric signal; first amplifying means foramplifying the electric signal from said detecting means with apredetermined amplification factor and outputting the amplified electricsignal; second amplifying means for amplifying the electric signal fromsaid first amplifying means with a variable amplification factor andoutputting the amplified electric signal; display means for displayingan image of said subject based on the electric signal outputted fromsaid second amplifying means; and control means for offsetting theelectric signal outputted from said first amplifying means by apredetermined quantity and controlling the variable amplification factorof said second amplifying means based on the electric signal outputtedfrom said first amplifying means; said control means comprising:meansfor offsetting the electric signal outputted from said first amplifyingmeans by said predetermined quantity and controlling the variableamplification factor of said second amplifying means within one frame ofthe electric signal; means for generating a frequency map of intensitiesof light incident upon said detecting means; means for integrating saidfrequency map with respect to intensities of light incident upon saiddetecting means; means for converting said frequency map into partshaving frequencies greater than a predetermined level and intensitiesadjacent to each other; and means for classifying the parts into groupsaccording to intensities of light incident upon said detecting means.52. An imaging device according to claim 51, wherein said control meanscomprises means for offsetting the electric signal outputted from saidfirst amplifying means by (max-min)/2 where max represents a maximumintensity of incident light in the parts or the groups and minrepresents a minimum intensity of incident light in the parts or thegroups.
 53. An imaging device according to claim 51, wherein saidcontrol means comprises means for controlling the variable amplificationfactor of said second amplifying means in proportion to 1/(max-min)where max represents a maximum intensity of incident light in the partsor the groups and min represents a minimum intensity of incident lightin the parts or the groups.
 54. An imaging device according to claim 51,wherein said display means comprises means for displaying pixels in aselected one of the parts or the groups in a monochromatic gray scalemode and displaying pixels in the other parts or the other groups in acolor display mode.
 55. An imaging device according to claim 51, whereinsaid display means has a plurality of windows and comprises means fordisplaying pixels in the parts or the groups in said windows,respectively.
 56. An imaging device according to claim 51, wherein saiddisplay means has a plurality of display monitor units and comprisesmeans for displaying pixels in the parts or the groups in said displaymonitor units, respectively.